Processes for applying various coatings such as metallic, alloy, ceramic and composite to a variety of substrates to form component products using thermal deposition coating methods are known. Such processes are employed to improve properties of the substrate such as hardness, corrosion resistance, heat resistance surface porosity and the like. Exemplary thermal deposition coating operations include: high-velocity oxy-fuel spraying (HVOF) and high-velocity air-fuel spraying (HVAF), DC and RF plasma spray in air atmosphere, vacuum chamber and/or inert gas chamber, electric arc (twin and single wire) spraying, laser powder coating or cladding, transferred arc coating operations such as plasma transferred-arc powder coating and welding overlay deposition, and so forth.
Thermal deposition coating operations deposit a preheated and/or molten coating material onto the surface of a substrate. In the coating process, multiple passes of a thermal deposition head are made over the surface of the workpiece each pass depositing a layer of coating material. A significant amount of energy, typically manifested as heat, is required to thermally deposit the coating material onto the workpiece and a portion of this energy is at least partially carried to the workpiece. Improper temperature control during thermal deposition frequently, leads to coating and workpiece overheating, thermal degradation, and damaging thermal stresses due to a mismatch of thermal contraction coefficients between the coating and substrate surface. When damage occurs through overheating, thermal stress and the like, the resulting coatings may be poorly adhering, or even fractured.
Heat removal from the workpiece during thermal deposition coating is critical and one of the most popular ways of practicing heat removal during the thermal deposition coating operation is to introduce breaks in the process cycle so that the accumulated heat is dissipated to the surroundings. Cooling air jets are often used to offset the loss of process productivity due to such a practice but (a) air cooling is usually insufficient and (b) the oxygen along with residual moisture and hydrocarbons present in the cooling air often are detrimental to the quality of coating.
The search for effective heat removal methods in terms of coolants for maximizing coating quality in the resulting component and/or process productivity led to the development of refrigerated and cryogenic gas cooling. While cryogenic cooling methods offer a significant enhancement in the ability to remove heat fast, they are rarely used in the thermal deposition coating industry because of an even further increased difficulty, or a narrower margin for error, in controlling temperature, i.e., heat build-up and thermal uniformity within the workpiece during coating.
Representative articles and patents illustrating thermal deposition coating processes some including the use of cryogenic coolants are as follows:
Nuse, J. D. and Falkowski, J. A. Surface Finishing of Tungsten Carbide Cobalt Coatings Applied by HVOF for Chrome Replacement Application, Aerospace/Airline Plating and Metal Finishing Forum, Cincinnati, Ohio, Mar. 27, 2000, disclose the use of HVOF for the application of tungsten carbide coatings in nose and landing gear substrates for aircraft as a replacement for chrome based coatings.
Stokes, J. and Looney, L., HVOF System Definition to Maximise the Thickness of Formed Substrates, Proceedings of the International Conference on Advances in Materials and Processing Technologies (AMPT '99), Dublin, Ireland, 3-6th Aug. 1999, pp. 775-784, disclose the use of HVOF to apply alumina-calcia stabilized zirconia deposits, CoNiCrAlY deposits and carbide deposits using carbon dioxide as a coolant. The effects of spray distance and forced cooling were determined.
Lucchese, P., et al., Optimization of Robotic Trajectories in the Atmosphere and Temperature Controlled Plasma Spray Process on Ceramic Substrate Using Heat Flow Modelling”, Proceedings of the 1993 National Thermal Spray Conference, Anaheim, Calif., 7-11 Jun. 1993, pp. 231-239 disclose the use of atmosphere and temperature controlled plasma spraying using liquid argon as a coolant. A refractory powder was sprayed on a rotatable ceramic workpiece with a robotic trajectory. A recording IR camera positioned in the plasma spraying area was used to measure temperature verses time and use those results to reduce heat fluxes and avoid substrate and coating destruction.
U.S. Pat. No. 6,740,624 B1 and EP 0 960 955 A1) disclose a method for providing a coating of metal oxides onto a substrate at thicknesses of generally greater than 5 mm by flame or plasma spraying. Cryogenic cooling of the back side of the substrate is performed during thermal spraying. The use of a single-point infrared sensor associated with a single-point cryogenic coolant source is suggested with the additional option of multiplying such sensor-cryogen source couples over the substrate surface.
U.S. Pat. No. 6,648,053 B2, WO 02/083971 A1, WO 02/083972 A1 and EP 1 038987 B1, disclose the use of coolant-free, sensed surface temperature-based thermal control methods and apparatus for an electric arc-spray-forming of thick deposits (billets) in an automated spray cell using a thermally insulating ceramic substrate, characterized by reduced distortion and internal stresses. The disclosed surface temperature sensing is based on real-time, two-dimensional mapping of spray-deposited surface using a multi-point measurement, thermographic or thermo-imaging (thermo-vision) camera.
The process control algorithm synchronizes the thermographic camera coordinates with the robotic sprayer coordinates, and when hot spots develop on the surface of the workpiece such hot spots are eliminated by controlling the amount of material sprayed on these hot spot areas by manipulation of the traverse speed and positioning of the robotic spray-forming gun.